Bullet Punch > Extreme Speed simply because you can now hit Pokemon like Gengar who assume they are immune to your priority and STAB move. I suppose you could say hitting Fairies is nice, but the utility of turning checks (let's be honest, no one brings Talonflame into Lucario unless it is absolutely sure it has a safe switch and wouldn't die to ESpeed (Rocks in this case making boosting pointless, should they be up.)
Another key point I think people need to realize is: ExtremeSpeed is weak. STAB CC outdamages it even when resisted. Priority on Lucario is used to beat things that would be able to revenge kill it otherwise if they are weakened enough. If ESpeed can not beat something at +2 (and yes many things are faster then Lucario so its very important) what good is it? Even though it has lower power, Bullet Punch is better barring Talonflame, but that is easily taken care of by either SR + CC or simply teammates.
Lucario has dropped in viability because Pokemon like Azumarill, while they can not directly OHKO with their priority, can take a hit and then kill it off. Ice Punch Lucario still loses to Landorus-T with the right EVs. Crunch still loses to most bulkier Ghosts (especially if they come in after a kill) as they carry Will-O-Wisp.
tl;dr: BP > ESpeed cause it's weak anyway and should be used for utility and Licario is worse because it's weak and new Pokemon wall it.
re: Iron Tail: no, it has some use, but leave the few relevant Fairies to teammates and try using your best coverage and utility to beat what you can.
as for the guy above me: you can't count SR because Clefable has Magic Guard gg Luke.